Both electronic and optical circuits are widely used, e.g. in information transmission systems. Electronic circuits are typically very small but their operation speed is limited, whereas optical circuits are extremely fast but their sizes are limited by diffraction. An integration of optics and electronics would combine the huge optical bandwidth with the compactness of electronics.
Photodetectors are devices to bridge optical and electric circuits. Traditional photodetectors are usually used to convert dielectric optical signals to electronic signals. They generally have low photoresponsivity per unit volume and hence require a relatively large size. Surface plasmon polaritons (SPPs) have been applied to improve the photoresponsivity of metal-semiconductor-metal photodetectors. However, the size of the photodetector is still in the micrometer scale.
More recently, a nanoparticle-formed cavity has been used to concentrate light into a subwavelength volume. This makes it possible to develop the photodetector in nanometer scale. Typically, two nanoparticles are used as a nanoantenna and form a nanocavity in between, and two additional pieces of metal are used as the electrodes of the detector. The electrodes are perpendicular to the nanoantenna. The waveguide, where light is injected, is perpendicular to the plane defined by the electrodes and the nanoantenna. However, such photodetector still works with dielectric waveguides, which is still limited by diffraction. in addition, fabricating such system (waveguide and detector) may be very challenging.
A need therefore exists to provide a plasmonic detector that seeks to address at least one of the above problems.